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Current Applied Physics 11 (2011) S114eS116

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Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Spatial structure of plasma potential oscillation and ion saturation currentin VHF multi-tile electrode plasma source

Kevin Ryan a,*, David O’Farrell a,b, A.R. Ellingboe a,b

a Plasma Research Laboratory, School of Physical Sciences and NCPST, Dublin City University, Collins Ave, Dublin 9, Irelandb Phive Plasma Technologies, Ireland

a r t i c l e i n f o

Article history:Received 25 July 2010Accepted 3 May 2011Available online 14 May 2011

Keywords:PECVDHigh frequencyCapacitive ProbePlasma potential oscillationIon saturation current density

* Corresponding author.E-mail address: kevin.ryan9@mail.dcu.ie (K. Ryan)

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2011.05.003

a b s t r a c t

Increasing RF frequency in the VHF range in the PECVD of Silicon for photovoltaic applications enablesa higher rate of deposition while maintaining film quality. However, plasma uniformity on largesubstrates is compromised as wavelength effects are encountered at high frequencies. Using a segmentedelectrode consisting an array of individual tiles each 180� out of phase with adjacent tiles, a scalable VHFsource that is not affected by wavelength effects is achieved. Due to the 180� phase shift between tiles,a spatial structure is imposed on the plasma. In this paper the spatial structure of the plasma potentialoscillation and the ion saturation current density are examined. A capacitive probe measures theoscillation of the plasma potential. Spatial profiles of the plasma potential oscillation exhibit peaks on tilecenters at odd harmonics (1F and 3F). The measured even harmonic (2F) is small in amplitude, asexpected for the differential power feed, and peaks at tile boundaries. Spatial profiles of ion saturationcurrent show peaks in plasma density over the tile-to-tile boundaries, with non-uniformity increasingwith RF power. A discussion of the measured profiles at a variety of powers and pressures is presented.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Capacitively coupled radio-frequency glow discharges arecommonly used for the deposition of materials by plasma-enhanced chemical vapor deposition (PECVD). Deposition overlarge area substrates by PECVD processes is of particular interest tothe photovoltaic industry. It has been shown that by increasing theRF frequency in the VHF range more power can be coupled into theplasma and thus an increase in the rate of deposition is observedwhile at the same time maintaining desirable material quality [1,2].However as substrate sizes increase, quarter-wavelength effects areencountered as the geometry and size of the electrode and chamberare now comparable to the wavelength of the driving RF voltage. Asa result of these quarter-l effects, a significant degradation of solarcell performance is observed [3]. For example deposition unifor-mity, band gap and cell efficiency all suffer a drop in quality. Toenable high frequency, large-area application, the use of multi-tilesystems with neighboring tiles out of phase has been proposed [4]Employing fully differential power results in zero net current intothe plasma and prevents wavelength effects. In this way a scalableplasma source that is not affected by wavelength effects can be

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achieved. However, this solution imposes a spatial structure ontothe plasma. This spatial structure contributes to power depositionin the system and can cause spatial non-uniformities in the plasmadensity. Spatially resolved measurements of plasma potentialoscillation and ion saturation current presented in this papercontribute to understanding the RF currents and power depositionin multi-tile systems.

2. Experiment

2.1. Plasma source

The system used in this investigation is the ‘Pastis’ plasmasource. The rectangular system resembles an asymmetric diode inwhich the powered electrode is replaced by a 3 � 4 array of tiles.The aluminum tiles are 10 � 10 cm2, with 1 cm wide inter-tiledielectrics made from alumina. A cross section along the 4-tiledirection is shown in Fig. 1(a). The resulting plasma volume is47 cm by 35 cm by 7 cm high. Gas is introduced into the plasmavolume via a showerhead structure in the tiles, and pumped bya turbo pump connected to a pumping plenum located behind thepowered electrode, with gas flowing from the plasma volume to thepumping plenum via gaps between the end-dielectrics andthe vacuum chamber (see Fig. 1(a)) and through 1 mm slits in the

Fig. 1. a) Cross section of the ‘Pastis’ plasma source showing the tile configuration ofa multi-tile segmented electrode glow discharge source, and b) Schematic of capacitiveprobe in a re-entrant quartz tube that is inserted into the plasma.

K. Ryan et al. / Current Applied Physics 11 (2011) S114eS116 S115

alumina inter-tile dielectrics. An Advanced Energy Ovation 35162generator provides RF power at 162 MHz. The output of the RFmatchbox feeds the PSTLD power splitter [4,5] which provides fullydifferential RF power to the tiles.

While the position of a potential substrate is shown, nosubstrate is present during these measurements. In addition,a 100 mm diameter observation window is located in the ‘ground’side, centered on the boundary between the two tiles on the right.Effects from this anomaly are seen in the capacitive probe data.

Fig. 2. Plot of the phase and amplitude of the fundamental in vacuum(a), in an argonplasma(b) and comparison of the amplitudes of the fundamental, second and thirdharmonics(c).

2.2. Probe diagnostics

Probes are inserted into the plasma by means of a feedthroughlocated on the end of the chamber aligned with the middle of thecentral row of tiles. This allows them to be manipulated across theface of the segmented electrode at a constant separation of 5 cmfrom the tile faces.

The capacitive probe consists of a coaxial cablewith a cylindricalcopper tip (5 mm diameter, 10 mm long) attached to the innerconductor. A small 50U resistor connects the inner core to the outerconductor as shown in Fig. 1(b). Constructed in this fashion, theprobe has a frequency response beyond 1 GHz [6]. The probe isplaced in a re-entrant quartz tube located in the plasma and is freeto move along the length of the quartz tube. Due to concernsregarding the plasma coupling to the outer conductor as well as theprobe tip, a semi-rigid triax cable was used in this experiment. Theouter conductor was grounded to the chamber at the probe feed-thru thereby shielding the two inner coaxial line from theplasma, leaving only the probe tip exposed.

The oscillation of the plasma potential couples to the exposedprobe tip. As a result, the measured voltage across the 50U resistorin the probe provides us with a representation of the plasma

potential oscillation. This measured signal consists of the funda-mental RF frequency (162 MHz) and second and third harmonics(324 MHz, 486 MHz). Discrete high pass and low pass filters wereused to select and/or isolate individual harmonics. Data is acquiredusing a Tektronix scope connected to a PC running LabView viaa GPIB connection for the capacitive probe.

Fig. 3. Plot of Ion Saturation Current Density vs Position for varying powers at400 mTorr.

K. Ryan et al. / Current Applied Physics 11 (2011) S114eS116S116

Langmuir probes have long been used as a diagnostic tomeasure the ion saturation currents in a plasma [7]. In this exper-iment, the probe is given a negative bias of �27 V to repel electronsand collect positive ions. The length of the probe is 104 cm witha 1.9 MHz low pass filter at the vacuum feed-thru. The collectionsurface is a single-sided planar probe with a 2.38 mm radiusoriented to face the tiles. The ion saturation current is obtained bymeasuring the voltage across a 10 kU resistor, connected betweenthe �27 V supply and ground, in series with the probe.

Because the charged particle density is directly proportional tothe the ion saturation current density, Ji ¼ e nsmBA/m2, if we assumethe electron temperature to be (approximately) constantthroughout the discharge, we can obtain a good approximation ofthe charged particle density as a function of position.

3. Results

3.1. Plasma potential oscillation

Fig. 2 plots select measurement of the plasma potential oscil-lation versus position across the source. Fig. 2(a) plots the ampli-tude and phase at the fundamental with no plasma present. Theamplitude shows the 4-lobe pattern of the tiles, with 180� phaseshifts at the tile boundaries. The amplitude imbalance between theright and left sides is believed to be due to the observationwindow(which straddles the tile boundary between the tiles on the right)which lowers the electric field strength on the right.

Fig. 2(b) plots the amplitude and phase at the fundamental withplasma (100W, 7mTorr Argon). The 4-lobe pattern is, again, seen inthe amplitude aligned with the tiles, but the nodes are not as deep.The phase data shows approximately 180� phase shifts betweentiles, but the transition is not as sharp, which is consistent with theshallow node structure in the amplitude data. In these condition,low power and low pressure, the plasma density is low. The sheath-currents of adjacent tiles couple via conduction in the plasmaand via the ground plane at the opposite sheath. The smooth

phase-variation with position is indicative of a combination of thetwo current paths.

In contrast to the vacuum data, the amplitudes are greater onthe right side of the system. This is, however, due to the same causee the observation window. With plasma the resulting plasmapotential oscillation is higher in the location where the plasma hasless contact with the grounded surface.

Fig. 2(c) plots the amplitudes of the first three harmonics versusposition. The 1f signal exhibits the 4-lobe pattern aligned with thetiles. The 2f signal is substantially smaller and exhibits amplitudeminima at the tile faces with maxima at the tile-to-tile boundaries.The phase shift of the second harmonic compared to the funda-mental is less prevalent, however a slow smooth change of phaseacross the chamber has been observed, indicating that the secondharmonic has less spatial structure. The 3f signal, although lessprominent than the first and second, appears to be at a maximumabove tile centers and at a minimum above boundaries. Phase data,follows the pattern of the fundamental harmonic and showsa phase shift of approximately 180� between tiles as expected.

3.2. Ion saturation current density

Fig. 3 plots the ion saturation current density as a function ofposition. Peaks in the ion saturation current density are found atthe tile boundaries while it is at a minimum at tile centers. Athigher powers and at higher pressures this spatial structurebecomes more apparent.

4. Conclusion

The spatial structure imposed on a plasma by a segmentedmulti-tile electrode has been demonstrated by investigation of theplasma potential oscillation and the ion saturation current densityusing capacitive and Langmuir probes respectively. Results showthat there is a clear correlation between the both the phase andamplitude of the plasma potential oscillation and the ion saturationcurrent density and the electrode configuration as expected.Changing both the power and pressure will change the spatialstructure of the plasma as theway inwhich the delivered couples toelectrodes and to the grounded chamber changes from a capacitiveto an inductive mode.

References

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